Power Over Ethernet (PoE) refers to the technology of providing IP-based terminals, such as an IP phone, an Access Point of WLAN, a web camera, etc., with DC power supply, while transmitting data signals for them, without changing the wiring fundamental structure of the existing Cat.5 of Ethernet. PoE technology can guarantee the proper operation of the existing network while making sure that the existing structuring wiring is safe, and cut down the cost to the utmost extent.
PoE is also referred to as Power over LAN or Active Ethernet, sometimes also referred to as Ethernet powering. It is the standard specification of transmitting data and electrical power at the same time utilizing the Ethernet transmission cable in the existing standard, and maintains the compatibility with the existing Ethernet system and users. IEEE 802.3af standard is a standard base on POE, it adds the standard related to powering directly over network cable on the basis of IEEE 802.3, and it is an extension of the existing Ethernet standard. And is the first international standard about power supply distribution.
A complete Ethernet power supply system comprises the power sourcing equipment (PSE) and the powered device (PD), the contact between both of them is established based on IEEE802.3af standard for the information about equipment type, consumption level and connection situation about the powered device, etc, based on which the power sourcing equipment controls to power the powered device over Ethernet.
The topology of a typical Ethernet power supply system is shown in FIG. 1. A power sourcing equipment 1, for example a hub with power supply, provides the twisted pair of LAN with power supply. At the end of the twisted pair, the power supply is used to drive the powered device 2, including an IP phone, a wireless access point, a camera and other devices.
Now referring to FIG. 2A-2C, methods for providing the PoE compatible powered device with DC power using the Ethernet transmission cable are shown respectively:
A method is “End-Span” (or endpoint), comprising the PoE switch/hub with PoE function installed inside, as shown in FIGS. 2A and 2B, wherein, two methods are comprised, method A is shown in FIG. 2A, wherein the power transmission uses the idle pin that are not transmitting data; the method B is shown in FIG. 2B, the DC current is transmitted over the cable that is used to transmit data, since the power transmission and the data transmission share the same cable, configuring a specific cable for power transmission is no longer necessary.
Another method is “Midspan”, as shown in FIG. 2C, for inserting a device having PoE function between the traditional Ethernet switch and the powered device, the inserted device applies method A, and the power transmission uses the idle pin that are not transmitting data, such as the power sourcing equipment 1 in FIG. 2C.
For the power sourcing equipment, the procedure of the power supply in the Ethernet can be described as follows:
1. Detection
When it starts, the power sourcing equipment outputs a very small voltage on the port, until it detects the end of its cable is connected to a powered device supporting IEEE 802.3af standard.
2. Powered Device Classification
When powered device is detected, the power sourcing equipment may classify the powered device, and evaluate the power consumption of the powered device.
3. Power Up
During a start up period with a configurable time (normally less than 15 μs), the power sourcing equipment starts from a low voltage to power the powered device, until it provides normal working voltage, for example 48V DC power supply.
4. Operating
The power sourcing equipment provides the powered device with stable and reliable 48V DC voltage, so as to satisfy the power consumption less than 13 W of the powered device. It may provide power up to 20 W-40 W in the PoE plus system.
5. Power Down
If the powered device is taken out of the network physically or electrically, the power sourcing equipment will stop powering the powered device quickly (normally within 300-400 ms), and start the detection procedure to detect whether there is a powered device connected to the end of the cable.
During the whole procedure, some events, such as over load, short circuit of the power consumption of the powered device, exceeding the power supply load of the power sourcing equipment, may cause interrupt to the whole procedure, then the power sourcing equipment will start from the first step of detection procedure.
According to the description above, if the powered device has no external power supply, the power sourcing equipment needs to power it until the powered device is taken out of the network physically or electrically.
It can be understood that the capacity of the Ethernet power supply system depends on the supplying capability of the electrical quantity of the power sourcing equipment and the maximum power consumption of each powered device.
Using the following parameters as example, the capacity of the Ethernet power supply system is analyzed in detail:
i) the supplying capability of the electrical quantity of the power sourcing equipment is 1000 W (Watt),
ii) the standby power consumption of powered device is 1 W;
iii) the maximum power consumption of the powered device is 100 W.
If all the powered device are in standby mode, the power sourcing equipment may support 1000/1=1000 powered devices; if all the powered device are in the maximum power consumption mode, the power sourcing equipment may only support 1000/100=10 powered devices. Thus, considering the security factors, if all the powered device are in the maximum power consumption mode, the number of the powered devices that can be supported by the system, namely the capacity of the Ethernet power supply system is only 10.
Using a phone system as an example, wherein, the powered devices are all IP phones. According to the experience, not all the phones are under calling mode at the same time, namely not all the phones are in the large power consumption mode. Usually, there are only 30% phones used at the same time, and the duration of most of the callings is less than 5 minutes. If the power sourcing equipment 1 provides the powered device 2 with power according to the situation that all the powered device are in the maximum power consumption mode, the number of the powered device that can be supported by the power sourcing equipment 1 is quite limited.
In order to extend the number of the powered device that can be supported by the Ethernet power supply system, if solely the supplying capability of the electrical quantity of the power sourcing equipment is enlarged, it will make the design of the power sourcing equipment more complicated, thus the cost will be increased; solely enlarging the supplying capability of the electrical quantity of the power sourcing equipment also needs to solve the problem of cooling; if external DC/AC power supply is used, the advantage of Ethernet power sourcing equipment will be lost. Thus, how to enlarge the capacity of the Ethernet power supply system effectively is a problem demanding prompt solution.